As used herein, the term "prosthesis" means any device that is implanted into the body of a human or animal to replace or supplement a body part. The term "soft tissue" refers to pliable, soft biological tissues such as muscle, fat and skin.
Implantable prostheses for correcting contour defects and form limitations in the human body have been in use since at least the early 1950's. The procedure has been frequently used for breast reconstruction, either following traumatic injury or surgical loss of the breast tissue, as through radical mastectomy, or to correct developmental hypoplasia. The procedure has also been used for cosmetic breast augmentation. Although the composition of the present invention can be used for reconstruction of any soft tissues, the discussion which follows is directed to reconstruction and augmentation of the breast tissue.
Early implants typically consisted of a foam or sponge material. These implants had two critical disadvantages. First, body tissue eventually invaded the sponge material, forming a hard scar which was painful and affected the consistency of the breast. The second disadvantage was that the sponge implant itself did not have the feel of a natural breast.
Later, breast prostheses were introduced comprising flexible sacs or lumens containing some type of soft gel filling. For example, an early breast prosthesis having this structure is described in U.S. Pat. No. 3,293,663 to Conin. The device disclosed in U.S. Pat. No. 3,293,663 comprises a silicone rubber molded flexible cup having a flat back and containing silicone gel. Silicone gel was used because it approximated the consistency of breast tissue and was thought, at the time, to be relatively non-toxic.
The combination of a lumen containing a substance approximating the consistency of breast tissue is still being used today. The lumen or sac is generally comprised of silicone rubber, Teflon, dacron, polyurethane or other plastic prosthesis materials. Typically such an implant has consisted of a flexible outer shell member made from silicone rubber, polyurethane or other known durable biocompatible polymer which has an elastic memory, and which is configured to a shape dedicated to provide the desired bodily contour. The generally hollow shell member or lumen may consist of a single layer or numerous layers in which one shell is placed inside the other. A variety of structural changes have been made to the early implants in an attempt to approximate the consistency and appearance of the human breast and to avoid the negative biological affects associated with introducing a foreign structure into the body.
For example, U.S. Pat. No. 4,573,999 to Netto describes a prosthetic implant for human breasts wherein the exterior surface of the lumen has concentric waves which serve to relieve the pressure caused by capsular contracture without reducing the volume or firmness of the implant. U.S. Pat. No. 4,790,848 describes a multiple lumen breast prosthesis in which the innermost lumen is spherical and free-floating to better approximate the natural movement of breast tissue. U.S. Pat. No. 4,650,487 to Chaylassian describes a multilumen breast lo prosthesis in which the lumens are filled with various substances to allow adjustment of the volume of the prosthesis.
The various lumens currently used are subject to a variety of mechanical breakdowns. For example, shell breakdown and rupture often occurs when the filler is a poor lubricant. This type of bag wall failure is known as "fold flaw." Several theories have been advanced to explain why the lumen ruptures. Fold flaw has been attributed to the stress differential between the inner and outer surfaces of a bag wall section and a creased area caused by one wall surface being in compression and the other being in tension. Fold flaw has also been attributed to friction between the opposing walls and changes in the physical properties of the lumen over a prolonged period of maintaining a crease. In any case, mechanical failure of the lumen results in introduction of the interior substances into the body cavity.
Shell rupture and breakdown can also occur by exterior mechanical manipulation of the breast. Such manipulation is sometimes necessary to rupture the capsule which naturally forms around the implant when an organism attempts to protect itself against invasion be a foreign body. Typically, an implant will be rapidly encapsulated by a fibrous structure composed primarily of collagen and glycosaminoglycans and containing fibroblasts and histiocytes. The capsule then contracts, resulting in hardening and spherical deformation of the implant and the surrounding tissue. This process is known as capsular contracture. The capsule becomes painful and deformed in appearance and usually must be ruptured or broken when it develops. The mechanical process of rupturing the capsule can cause the implant itself to rupture and its contents to leak into the body cavity.
Various attempts have been made to avoid the development of capsular contracture. For example, U.S. Pat. No. 4,772,285 describes a collagen coated soft tissue prosthesis for that purpose. However, these methods have been unsuccessful and the risk of leaks and ruptures remains high. Thus, it is critical that the adverse biological effects of the filler material be minimal.
U.S. Pat. Nos. 4,740,208 and 4,840,628 to Cavon describe cast, uncontained, silicone gel elastomer implantable prostheses. U.S. Pat. No. 4,657,553 describes a soft tissue implant composed of a hydrogel filler comprising a gellable polysaccharide and/or a protein or polypeptide, and a polymer of a hydrophilic acrylic and/or methacrylic acid derivative. U.S. Pat. No. 4,772,284 to Jefferies et al. describes a biocompatible material suitable for implant use which comprises a purified reconstituted collagen gel and a purified gel of alpha-amino acid homopolymers or random copolymers. U.S. Pat. No. 5,067,965 to Ersek et al. discloses a bio-osmotic gel for filling implant lumens with improved radiolucence consisting of a bioosmotic gel containing polyvinyipyrolidone and a biocompatible salt.
Many different substances have been used to fill the lumen of the implant. Examples of these include normal saline, foam pads, and silicone oil or silicone gel. Each of these prior materials, however, exhibit at least one major drawback. Saline, for example, is a poor lubricating agent and prostheses filled only with saline have been known to undergo accelerated shell breakdown and rupture due to friction of the inner shell rubbing against itself (fold flaw). Molded foam inserts were found to rapidly calcify after implantation in the body and do not feel or look like natural breast tissue.
After a considerable period of time, silicone oil remains the most commonly used filling material. With respect to most implant prostheses, including breast implants, the filler is normally utilized in the form of a partially vulcanized silicone which is sealed inside the lumen prior to implant. This material has enjoyed a long period of use principally because of two desirable properties. Silicone oil is a natural lubricant and this tends to prevent shell breakdowns occasioned by internal friction. In addition, the viscosity of silicone oil placed inside a partially inflated lumen imparts a consistency to the structure which closely predicates natural breast tissue.
Recently, the use of silicone in prostheses has been severely limited in the United States by the Food and Drug Administration due to the lack of safety data regarding its use and an increase in awareness of adverse biological effects caused by leakage of silicone into the body cavity. The use of silicone oil and gel presents several major disadvantages. One such disadvantage involves the inability of the body to eliminate silicone oil. The silicone oil continually migrates or leaches through the wall of the implant into surrounding tissue where it is not eliminated from the body. The silicone oil can accumulate and produce a painful inflammatory reaction. Silicone oil may also be associated with the formation of autoimmune disease. N. Touchette, "Silicone Implants and Autoimmune Disease: Studies Fail to Gel", J. NIH RESEARCH, Vol. 4, pp. 49-52 (May 1992). In the case of traumatic rupture of the shell, silicone is forced into the surrounding tissues, traveling down the facial planes where it causes a severe foreign body reaction and requires extensive surgery to remove. An additional drawback with respect to silicone oil is that it is radiographically dense. This makes it more difficult to examine the area of implant by X-ray, or the like. This may obscure mammogram detail and delay detection of breast cancer.
Thus, what is needed is a composition for filling the lumen of a prosthesis that has the consistency and feel of natural tissue and, that is non-toxic so that if the prosthesis should leak or burst, the composition will not cause any adverse biological effects. What is further needed is a composition for filling the lumen of a prosthesis that is radioluscent so that it does not interfere with x-ray detection of abnormalities in tissues surrounding the prosthesis. The composition should also serve to lubricate the interior of the prosthesis to avoid mechanical breakdown of the shell of the prosthesis. What is further needed is a composition that would be resistant to the formation of a fibrous capsule leading to contracture and possible rupture.